There are many reasons why one would want to be able to detect or measure, or both, fluids which are flowing at very low rates. Those rates may be as low as drops per minute or drops per hour. Examples of such low flow systems includes intravenous (IV) applications of nutrients or medicines to humans or animals. Another would be in a laboratory where drops of a liquid are added to another substance for a variety of purposes. Pesticides, insecticides or herbicides may be added to irrigation water at a very low rate.
As another example, increasing environmental awareness and respect for valuable natural resources has created an abiding concern in the area of underground leak detection. This is especially true with regard to fuel tank storage and dispensing systems. The industry has been continuously in search of new technologies and enhancements to existing technologies for underground leak detection, particularly pipeline leak detection.
Conventional leak detection methods for pipelines involve pressurizing the line and then observing a pressure decay when there is a leak. This method is subject to false readings due to temperature changes of the pressurized liquid caused by heating or cooling from the environment. A small temperature change, either a reduction or increase in temperature, can cause a large change in the pressure of liquid, which is, for the most purposes, incompressible. In pipelines, where a relatively few number of gallons of liquid may be involved, a very small amount of liquid volume, that is, flow, causes a large change in pressure. For example, a typical gasoline station may have 120 feet of pipe connected to one storage tank. There may be about 20 gallons of fuel in that length of pipe. A one-degree F change in liquid temperature at a typical temperature coefficient of expansion results in a detectable change in pressure. However, that one degree of temperature change results in a volume change of only 0.014 gallons of liquid.
Attempts to use currently available flow meters or flow sensors have been satisfactory for some purposes but they are insufficiently accurate to detect fluid flow at the minimum level required by the United States Environmental Protection Agency with respect to fuel storage and dispensing systems. This minimum level is currently 0.1 gallon per hour and any leak detection system for either fuel tanks or piping which does not achieve this minimum detection level will not be acceptable.
The currently available flow meters known to applicant all leak to some extent. Typically that leak rate may be in the range of at least one drop per second, which calculates to about 0.05 gallons per hour. That rate would be undetectable for those known flow meters. This is unacceptable for the very low flow applications mentioned above. Particularly with respect to the pipeline example, the apparent leak rate of 0.014 gallons per hour, if the one degree F change took place over the course of one hour, would be undetectable by a conventional flow meter. By way of further example, a leak rate of 0.1 gallon per hour amounts to only two drops per second. That is below the threshold leak rate detectable by currently available flow meters which themselves often leak at a rate of more than two drops per second.
A flow meter is a key element in a leak detector of the type mentioned above. If the flow meter can detect the extremely small flows which are mandated by current regulations for the fuel industry, it can enable a detection system to operate at the desired level of efficiency and sensitivity. Just as important are the uses for drip applications in the fields of medicine, science and agriculture, to name a few, where very low flow rates need to be detected and controlled.
Many flow meters currently available employ either small fixed calibrated orifices or have a sliding (piston) seal, or both. While these may sometimes detect the extremely low flow rates of interest, the small orifices or sliding surfaces are subject to being clogged by substances in the fluids passing through, such as varnishes in gasolines, or dirt particles in any fluids, thereby eventually making those flow meters inoperative.